1. Field of the Invention
The present invention relates to an improved radio channel setting control scheme, and, in particular, to a radio channel setting control scheme for controlling setting of radio channels used for communications between a base-station apparatus and a mobile-station apparatus in a mobile communications system employing a CDMA scheme.
2. Description of the Related Art
According to a mobile communications system such as a cellular-phone system widely spread recently, the whole service area is divided into relatively small radio zones called cells. Such a mobile communications system includes a plurality of base-station apparatuses which cover respective divided radio zones, mobile-station apparatuses which communicate by setting up radio channels among these base stations, and radio network control apparatuses. Each radio network control apparatus can control a plurality of base-station apparatuses. However, in case very large number of base-station apparatuses exist in a mobile communications system, a plurality of radio network control apparatuses may be provided.
In a CDMA (Code Division Multiple Access) scheme employed between a base-station apparatus and a mobile-station apparatus by which user information is spread into a wider band radio spectrum in proportion to the transmission rate thereof, each radio channel is provided by a spread code. As each user has a different spread code, many users can share a same radio frequency band. A plurality of radio frequency bands each being shared by a plurality of users may be used in case a much increased number of users exist.
Usually, user information is of dozens of kbps through hundreds of kbps, and, according to the CDMA scheme, the user information is spread into a band width of several MHz by the above-mentioned spread code. For example, according to a W-CDMA (Wideband CDMA) scheme, for which a standard has been decided in 3GPP (The Third Generation Partnership Project), the rate (chip rate) by which a bit pattern of a spread code is repeated is 3.84 Mcps.
The specific method of application of the spread code according to W-CDMA is defined in a specification (TS25.213 “Spreading and Modulation (FDD)”) provided by 3GPP. The spread code includes a scrambling code allocated for every base-station apparatus for a long period, and a channelization code used in common by all the base-station apparatuses for a short period. A signal transmitted from each base-station apparatus is spread using both the scrambling code and the channelization code.
A plurality codes are previously defined for the scrambling code in the whole system. A system designer selects and allocates codes of these codes to be allocated to each base-station apparatus. Since the allocation design is made such that different scrambling codes may be allocated for the respective base-station apparatuses, the radio channel of each base-station apparatus can be identified by its scrambling code.
On the other hand, different radio channels in the same base-station apparatus can be identified by the channelization codes. The channelization code is designed such that interference between the codes may be cancelled out, namely, such that code series are orthogonal each other, and the number thereof is limited.
When the scrambling codes differ, it is possible to identify, as different radio channels, even if they have a same channelization code. For this reason, when the scrambling codes differ, the same channelization code can be used. In fact, in a mobile communications system using the W-CDMA scheme, the same channelization code is actually used in each base-station apparatus. As mentioned above, since the number of channelization codes is limited, there is a possibility that all the spread codes are on use. For this reason, in case a spread code is allocated, the use situation of the spread code is always recognized, and actual code allocation should be made after examining as to whether it can be allocated, due to the limited number of codes.
By the way, since the CDMA scheme is as above-mentioned a scheme in which many users share the same radio frequency band, all the signals other than a self-communication wave act as interference. Although interference electric power naturally becomes larger as the number of users increases, in order to secure the self-communication wave in a predetermined quality, the ratio of the self-communication wave electric power to the interference electric power needs to be larger than a value by which the quality is maintained. That is, the interference electric power should not become larger without any limitation, and it should have a fixed limit. Accordingly, it can be said that there is a limit also in the number of users which can be accommodated by the system.
Japanese laid-open patent application No. 8-19148, ‘Call Acceptance Control Method and Apparatus’, and international publication No. WO98/30057, ‘Call Acceptance Method in CDMA Mobile Communications System and Mobile-Station Apparatus’ disclose arts in which a user accommodation situation is monitored, and it is determined as to whether or not a new call is to be accepted according to the CDMA scheme.
According to Japanese laid-open patent application No. 8-19148, determination is made as to whether or not a new call is to be accepted based on an estimation on interference power in the base-station apparatus. According to this publication, in an uplink circuit (directed from a mobile-station apparatus to a base-station apparatus) in a mobile communications system according to CDMA, the interference power is an important factor in the viewpoint of communications quality.
On the other hand, according to the international publication No. WO98/30057, information concerning uplink interference power (total of interference power in the direction from a mobile-station apparatus to a base-station apparatus) and information concerning downlink transmission power (total of transmission power in the direction from the base-station apparatus to the mobile-station apparatus) is reported, and, thereby, the mobile-station apparatus determines as to whether or not a new call is to be accepted. According to the publication, transmission power in the base-station apparatus is an important factor in the viewpoint of quality of communications, in the downlink circuit (directed from the base-station apparatus to the mobile-station apparatus) in the mobile communications system employing CDMA.
Thus, according to W-CDMA, user accommodation capability may be limited due to the uplink interference power and/or downlink transmission power. Accordingly, it is necessary to determine, after recognizing situation on these uplink interference power and downlink transmission power, whether or not a newly occurring call should be accepted, i.e., whether or not a new radio channel should be set up.
According to W-CDMA mobile communications scheme, in order that a radio channel is set between a base-station apparatus and a mobile-station apparatus, and communications services are provided therethrough, it is necessary to prepare a predetermined hardware device in the base-station apparatus for setting the above-mentioned spread code, performing error correction coding on user information, coding into the radio channel, and spread modulation/demodulation, also, for transmitting the modulated signal as a radio signal, and, also, for receiving a signal from the mobile-station apparatus.
Usually, this hardware device is released and thus can be utilized for another communications purpose after the user terminates the communications, or moves into another cell. As to what hardware device should be prepared in the base-station apparatus is designed depending on the traffic request in the cell, etc. For example, by using the Erlang B formula used conventionally, the number of hardware devices is determined such that a time ratio of occurring a situation in which, since all the hardware devices are on use, and, thus, no new radio channel can be set may be sufficiently small value (for example, several percents). Such a design scheme is disclosed by L. Kleinrock's work, “Queueing Systems” (John Wiley & Sons, 1975) etc. in detail, for example. Thus, it is necessary to always monitor the use situation of the hardware devices in the base-station apparatus, and to determine whether or not a new radio channel is to be set up based thereon.
However, in the mobile communications system according to CDMA, quality communication may be unable to be performed only by monitoring the use situation of spread code or hardware devices described above.
For example, assuming that if the traffic on the adjacent cell increases interference electric power increase in the self-cell. For this reason, there is a possibility in that, even if there are few users under communication within the self-cell and also there is a sufficient margin for spread code or hardware devices, since there is no margin in the radio resources, a new radio channel cannot be set up.
On the other hand, there is a possibility in that, when interference by the traffic in an adjacent cell is small and also the traffic in the self-cell increases, since there is no margin in spread code or in hardware devices even if there is a sufficient margin in radio resources, a new radio channel cannot not be actually set up.
Moreover, when a designer of the system estimates the traffic in the self-cell few, since the number of the hardware devices mounted in the base-station apparatus is insufficient even if there is a sufficient margin in spread code or radio resources, there is a possibility that a new radio channel cannot be actually set up.